Various wrapping materials have been developed and proposed to date for packaging of a variety of articles such as foods and beverages, drugs and the like. In particular, for packaging of foods and beverages, drugs and the like, there is a need for packaging that employs packaging materials comprising a gas barrier laminated body having an effect of blocking infiltration of oxygen or water vapor from the external air which promotes putrefaction or alteration, or in other words, having an excellent gas barrier property. Various forms of gas barrier films have therefore been developed and proposed.
Gas barrier films comprising polymer resin materials with gas barrier properties, such as polyvinylidene chloride, polyvinyl alcohol and ethylene-vinyl alcohol copolymer, or plastic multilayer films laminated on other plastic base materials, as well as those laminated with aluminum foil which has an excellent barrier property as the most common barrier material, and metal vapor deposition films having a metal element such as aluminum vapor deposited on one side of a plastic film, are used as gas barrier materials in packaging materials.
However, materials employing plastic films suffer considerable reduction in gas barrier properties after boiling treatment or high temperature, high humidity retort treatment, depending on the use of the packaging, and gas barrier properties comparable to those of metal foils or metal vapor deposition layers have not been achieved. Moreover, they have low environmental friendliness, as they fail to decompose in the natural environment when discarded after use, and when incinerated they produce air pollution or require high temperatures.
Metal vapor deposition films laminated with aluminum foil or vapor deposition layers, on the other hand, are excellent gas barrier films, but have poor transparency due to the metal foil or metal vapor deposition layer, while also having low environmental friendliness as they are poorly suited for incineration and are difficult to treat as waste after their use.
Thus, in order to meet demands for gas barrier properties, and especially maintenance of excellent gas barrier properties after boiling treatment or high temperature, high humidity retort treatment, gas barrier films have been proposed that are formed by laminating a vapor deposition layer comprising an inorganic compound as a first layer on a base material comprising a polymer resin composition, and as a second layer, a gas barrier coating film obtained by coating and heat drying a coating agent whose main agent is an aqueous solution including a water-soluble polymer, and at least one from among (a) one or more alkoxides and/or hydrolysates thereof and/or (b) tin chloride, or a water/alcohol mixed solution (PTL 1).
This gas barrier coating film forms a chain or three-dimensional dendritic polymer after hydrolysis of the alkoxide, and reaction with the water-soluble polymer on the molecular level causes formation of a complex (Si—O—C bond), whereby a gas barrier property and resistance to moist heat are exhibited.
However, when an alkoxide hydrolyzes, the alkoxide undergoes condensation reaction immediately after the hydrolysis, and as time progresses, formation of coarse particles (gel masses) occurs, the coarse particle formation of the hydrolysate and water-soluble polymer producing steric hindrance by phase separation between the two, such that the film quality has numerous gaps as a complex, making it difficult to obtain a satisfactory gas barrier property, while under the high temperature and high humidity of boiling treatment or retort treatment, and after retort sterilization treatment, the water-soluble polymer swells and exhibits a reduced gas barrier property, so that the gas barrier film cannot be considered adequate.
There has also been proposed a technique for preventing swelling of hydrogen bonds formed between the water-soluble polymer and hydrolysate, by forming the gas barrier coating film using a sol-gel method and, as a modification to the gas barrier film for an improved gas barrier property, coating a 3-component mixed solution comprising an alkoxide hydrolysate, a polyvinyl alcohol and a silane coupling agent including an epoxy group as the organic functional group, onto an inorganic compound layer, and heat drying it to create a network structure of organic functional groups (PTL 2) (see FIG. 2).
However, in a gas barrier coating film formed from such a 3-component mixed solution, the silane coupling agent added to the mixed solution causes steric hindrance in the coating film so that the gas barrier property of the film itself obtained with the same formulation is inadequate compared to one without addition of a silane coupling agent, and the reduction in the gas barrier property after retort treatment remains a problem.
Thus, no gas barrier film yet exists that exhibits an adequate gas barrier property under the high temperature and high humidity of boiling treatment or retort treatment.